Electric drives are becoming increasingly more common in the motor vehicle field, and are fulfilling more and more functions.
Because of their high performance and their high power density, synchronous machines are mostly used in the motor vehicle industry.
These are used in particular in hybrid electric vehicles (HEVs) and electric vehicles (EVs) in the electric power steering, air conditioners and cooling fans of the thermal engines.
FIG. 1 shows schematically an electric drive with a synchronous machine 1. A system of this type also generally comprises:                a source of energy constituted by a battery 2;        a static converter constituted by an inverter 3;        a control unit 4.        
An adequate power inverter 3 is required to generate the various voltages of variable frequencies and phase amplitudes from the battery 2.
The synchronous machine carries out the conversion between the electrical energy and the mechanical energy.
As indicated in FIG. 1, measurement sensors (direct voltage supply, phase currents 5 and angular position/speed of rotation 6) associated with a reference 7 (torque or speed) constitute the inputs of the control unit 4 of the system.
FIG. 2 shows schematically, in the case of a three-phase synchronous machine 1, the main elements of the control unit 4 which controls the power electronics of the inverter 3, in order to supply the machine 1 with the correct voltages corresponding to the references.
The purpose of this control unit 4 is to permit functioning with good performance in a wide range of torques and speeds.
For this purpose, closed loop control is necessary.
The current regulation 8 (Current Control Loop—CCL) depends on correction which ensures good control of the phase currents measured at a current reference iref.
A unit for calculation of the current reference 9 provides iref from the torque/speed reference 7 and the electrical parameters of the machine 1.
It transforms a mechanical reference into an electrical reference.
When the reference voltage Vref has been calculated by the current regulation unit 8, a vectorial pulse width modulation unit 10, or vectorial PWM (Space Vector Modulation—SVM) generates the duty cycles which must be applied to the arms of the inverter 3 from the components of the reference voltage in a Clarke reference.
Next, a power control unit 11 generates the control signals U, V, W used to control the inverter 3.
As far as the current regulation 8 is concerned, since a three-phase system is being used, the control can be analysed in a plurality of references.
The well-known Clarke transformation is a projection of the values relating to the three phases on two fixed axes (α, β). The Clarke transformation is then followed by rotation of the axes, which converts the alternating components of the reference (α, β) into direct components according to a direct axis and an axis in quadrature (reference (d, q)) relative to the magnetic flux produced by the rotor. The combination of the Clarke transformation and the rotation of the reference (α, β) to the reference (d, q) is known by the name of Park transformation.
The main advantage of these transformations is the reduction of the order of the system and decoupling of the control.
The control of machines of this type requires information 6 concerning the angular position and/or the speed of the rotor of the machine. Conventionally, the position/speed information 6 is obtained by one or two measurement sensors, in the knowledge however that a single angular position sensor can generally suffice, taking into account the fact that the speed of rotation can be obtained by calculation of the first derivative relative to the time of the angular position.
In order to reduce the cost of the synchronous machine and its control, it is known from document U.S. Pat. No. 5,569,994 to dispense with the position/speed sensor(s) in order to control the machine. For this purpose, the electromotive forces (emf) generated by the various phase windings of the machine are used in order to estimate the position of the rotor. However, this method according to U.S. Pat. No. 5,569,994 does not provide a satisfactory solution for measurement of the angular position of the rotor at low speed.
Whatever the case, when safe and accurate functioning of the electric drive is required, it is difficult to dispense with the position/speed sensor 14, 15, and the control unit 4 must be tolerant to a deficiency 12, 13 of the position/speed sensor 14, 15. This makes it possible to ensure functioning without interruption and in conformity with the safety requirements.
At present, in the most demanding applications, redundancy of the equipment is used to overcome a sensor malfunction, which makes the system more complex and increases its cost.
Concurrently, software redundancy is far more advantageous because of its capacity for development, as well as its low cost. For this purpose, “software sensors” as they are habitually known, are produced.
These use other available measurements, in particular those provided by current sensors, and other data relating to the control, in order to reconstruct the missing signal.
For example, document U.S. Pat. No. 7,002,318 describes a method and a device for control with malfunction tolerance of a vehicle propulsion system. In the case of failure of the sensor for the angular position of the rotor of the three-phase electric motor with permanent magnets, an estimation of the position is calculated according to the phase currents, and is used by the control device. It will be noted however that this document does not propose any satisfactory solution for a polyphase synchronous rotary electrical machine.